1. Field of the Invention
The present system relates to heating systems for motor vehicle batteries.
2. Description of the Related Art
In an electrically-propelled vehicle, one of the foremost challenges is to maximize the energy available from the on-board batteries to drive the electric motor or motors which propel the vehicle. Typically, many batteries will be connected in series, at a voltage of several hundred volts, to provide that energy.
In order to store and extract a reasonable amount of energy from the batteries, batteries of most technologies should be kept above a predetermined minimum temperature while being charged and discharged. Thus, in ambient temperatures below the predetermined minimum, the batteries should be heated. Further, in order to best balance the state of charge of the batteries when being charged and when being discharged, the temperatures of the batteries should be kept as close as reasonably possible to one another. A warmer battery will have greater capacity to accept and deliver energy than a cooler battery. But, an important fact to consider is that once one of the batteries is discharged, energy should no longer be taken from any of the batteries. Thus, the range of the vehicle is limited by whichever battery is least able to deliver energy. Since ability of the batteries to store and to deliver energy is related to temperature, the temperatures of batteries should be maintained as reasonably uniform as possible.
In one known system for heating the batteries of an electric vehicle, an individual electric heater is provided to heat each battery. The electric heaters are coupled together in series across the series connection of the vehicle's batteries. Although such a system may be generally effective, it has a disadvantage of having many high-voltage connections among the heaters. Those skilled in the art will recognize that the high-voltage connections may tend to find leakage paths to chassis ground. This is disadvantageous, in that electric vehicles are often designed to have the high voltage of the series connection of batteries isolated from chassis ground.
Electric vehicles often are also designed with a lower-voltage system which is referenced to chassis ground. This system powers the "conventional" electrical accessories of the vehicle (lamps, radio, and the like). One way of generating such lower voltage is via a DC-to-DC converter which converts the high voltage of the series connection of batteries to the lower voltage. While one may look to the lower-voltage system in order to power the heaters for the batteries, energy in the lower-voltage system is fairly scarce, due to the limitation of the DC-to-DC converter to generate the energy. Adding too many heaters to the low-voltage system can thus exceed the capability of the DC-to-DC converter to supply power to all of the loads connected to the low-voltage system.
Another challenge in the design of a heating system for electric vehicle batteries is in uniformly heating the batteries. Where there is a large number of batteries, which is fairly typical in order to provide the vehicle with the highest practical energy storage capacity, the batteries must be packaged into whatever space is available in the vehicle. This can result is the batteries being packaged in an irregular geometry. Such an irregular geometry may be difficult to uniformly heat. Further, the carrier in which the batteries may be located may have non-uniform heat loss characteristics in various areas. Such non-uniform heat loss characteristics can be due to wiring pass-throughs in the walls of the battery carrier as well as non-uniform insulation thicknesses in the walls of the battery carrier. The packaging space available in the vehicle for the battery carrier may necessitate such varying thicknesses of insulation in the walls of the battery carrier. The non-uniform heat loss may further be due to hydrogen-ventilation provisions in the battery carrier. For all of the above reasons, uniform heating of the batteries of the electric vehicle can be a challenge. It is even a greater challenge if no system is provided to circulate air within the battery carrier in order to help promote a uniform temperature within the battery carrier. Avoiding the use of such an air-circulation system can provide considerable cost advantages.
Thus, a battery heating system for an electric vehicle which can minimize high-voltage connections, which can avoid placing excessive demands on the lowervoltage system and which can assure uniform temperatures of the batteries without the need for an air circulation system will provide advantages over the prior art.